A need exists for additional methods to prepare 3-halo-4,5-dihydro-1H-pyrazoles. Such compounds include useful intermediates for the preparation of crop protection agents, pharmaceuticals and other fine chemicals.
Several methods have been reported for the preparation of 3-halo-4,5-dihydro-1H-pyrazoles. For example, J. P. Chupp, J. Heterocyclic Chem. 1994, 31, 1377-1380 reports the preparation of a 3-chloro-4,5-dihydro-1H-pyrazole by contacting the corresponding oxo-pyrazolidine with phosphorus oxychloride. M. V. Gorelik et al., Journal of Organic Chemistry U.S.S.R. 1985, 21, 773-781 (English language translation of Zhurnal Organicheskoi Khimii 1985, 21(4), 851-859) discloses the preparation of 3-chloro-4,5-dihydro-1H-pyrazoles by way of diazonium salt intermediates prepared from the corresponding 3-amino-4,5-dihydro-1H-pyrazoles. K. K. Bach et al., Tetrahedron 1994, 50(25), 7543-7556 discloses the preparation of a 3-chloro-4,5-dihydro-1H-pyrazole by dipolar cycloaddition of an acrylate ester with a hydrazidoyl chloride intermediate formed by decarboxylative chlorination of a hydrazone of glyoxylic acid using N-chlorosuccinimide. The need remains for alternative methods, particularly those of broad chemical structure generality and which use relatively low cost reagents commercially available in industrial quantities.